Optical test system and method for determining size of gap between two substrates of optical element

ABSTRACT

An optical test method is provided. The optical test method includes emitting light through a gap between two substrates of a tested optical element disposed on a holder to generate a plurality of light beams. The optical test method further includes driving the holder with the tested optical element to move to N positions. The optical test method also includes receiving one of the light beams from the tested optical element in the N positions to generate N first intensity signals. In addition, the optical test method includes determining the size of the gap of the tested optical element according to the N first intensity signals and reference data.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional PatentApplication No. 62/555,144, filed on Sep. 7, 2017, the entirety of whichis incorporated by reference herein.

BACKGROUND

Three dimensional (3D) optical imaging systems are capable of providingdistance measurements and a depth image of the objects within thecapture area. Such systems are currently used in gaming and multimediaapplications, for example, to provide human identification and gesturerecognition, as well as in a wide variety of other applications, such asthe inspection of semiconductors and other goods, computer-aided design(CAD) verification, robot vision, and geographic surveys. Generally, 3Doptical imaging systems include an optical pattern projection systemincluding a light source for illuminating objects. The 3D opticalimaging system further includes a light receptor such as a 3D camera forreceiving light reflected from an object and forming a 3D image of theobject from the reflected light.

In some applications, a diffractive optical element is used in theoptical pattern projection system for creating the desired projectionpattern. The structure of the diffractive optical element is related tothe optical characteristics of the diffractive optical element and theoptical pattern projection system. Although existing test systems andmethods for the inspection of the diffractive optical element havegenerally been adequate for their intended purposes, they have not beenentirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages of the present disclosure, reference is now made to thefollowing descriptions taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view of an optical pattern projection system, inaccordance with some embodiments.

FIG. 2 is a cross-sectional view of the diffractive optical element inFIG. 1, in accordance with some embodiments.

FIG. 3 is a schematic view of an optical test system for determining thesize of a gap between two substrates of an optical element, inaccordance with some embodiments.

FIG. 4 is a simplified flowchart of an optical test method fordetermining the size of a gap between two substrates of an opticalelement, in accordance with some embodiments.

FIG. 5 is a schematic view illustrating that the holder (with oneoptical element model) is moved to multiple positions during the test,in accordance with some embodiments.

FIG. 6 is a diagram plotting multiple sets of sample (second) intensitysignals that are corresponding to a number of optical element models(with different bonding gap sizes) in multiple positions, in accordancewith some embodiments.

FIG. 7 is a schematic view illustrating that the holder (with a testedoptical element) is moved to multiple positions during the test, inaccordance with some embodiments.

FIG. 8 is a schematic view showing that the size of a gap between twosubstrates of the tested optical element is determined according to themeasured first intensity signals corresponding to the tested opticalelement and the reference data including multiple sets of sample(second) intensity signals that are corresponding to a number of opticalelement models, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed. Various featuresmay be arbitrarily drawn in different scales for the sake of simplicityand clarity.

Furthermore, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

As described above, 3D optical imaging systems utilize an opticalpattern projection system to create a projection pattern to illuminatean object or objects as desired. FIG. 1 is a schematic view of anoptical pattern projection system 1 in accordance with some embodiments.As shown, the optical pattern projection system 1 includes a lightsource 2, a collimator lens 3, and a diffractive optical element 4.

The diffractive optical element 4 is disposed on one side of an object O(e.g., a screen). The light source 2 is a laser source for emitting alaser beam L1, such as a visible, infrared (IR), or other radiationwhich can be selected depending on different applications. The laserbeam L1 is modulated by the collimator lens 3 and a parallel collimatedbeam L2 is output from the collimator lens 3. The diffractive opticalelement 4 includes specified diffractive grating structures (see FIG. 2)formed therein. When the collimated beam L2 passes through thediffractive optical element 4, the collimated beam L2 is diffracted bythe diffractive grating structures.

Consequently, a number of diffracted light beams L3 with the desiredoptical pattern (e.g., a dot-array pattern, a striped pattern, etc.) areprojected onto the object O positioned at a specified distance from thediffractive optical element 4. The optical pattern of the diffractedlight beams L3 may be changed by providing or forming differentdiffractive grating structures in the diffractive optical element 4. Insome other embodiments, the collimator lens 3 may also be omitted.

FIG. 2 shows a cross-sectional view of the diffractive optical element 4in FIG. 1 in accordance with some embodiments. As shown, the diffractiveoptical element 4 includes a first substrate 5A and a second substrate5B parallel to and stacked on each other. In some embodiments, the firstsubstrate 5A and the second substrate 5B are located on both sides of anadhesive layer 8. Each of the first and second substrates 5A and 5B mayinclude a glass, polymer or another optional material that allows light(e.g., a visible, infrared, or other radiation) with a particularwavelength or in a wavelength range (used in the optical patternprojection system 1) to pass through.

In some embodiments, a first diffractive grating structure 6A and asecond diffractive grating structure 6B are formed on the opposingsurfaces S1 and S2 of the first and second substrates 5A and 5B,respectively. Each of the first and second diffractive gratingstructures 6A and 6B is designed according to the optical diffractiontheory (i.e., a phase-type optical diffraction structure). The first andsecond diffractive grating structures 6A and 6B may be formed by asemiconductor processing technology (e.g., including photolithography,etching processes, etc.) or another optional technology.

In some other embodiments, the first and second diffractive gratingstructures 6A and 6B may be formed on the surfaces of two additionalepoxy material layers (not shown), and the two epoxy material layers aredisposed on or adhered to the first and second substrates 5A and 5B,respectively. The epoxy material layers allow light (e.g., a visible,infrared, or other radiation) with a particular wavelength or in awavelength range to pass through.

In some embodiments, the configuration and arrangement of the firstdiffractive grating structure 6A may be the same as or different fromthe structure configuration and arrangement of the second diffractivegrating structure 6B, depending on the desired optical pattern of thediffracted light beams L3 to be produced for various applications. Theconfiguration (e.g., semicircle) of each structure unit in the first andsecond diffractive grating structures 6A and 6B and the arrangement(e.g., pitch) of the structure units are not limited to the embodimentsshown in FIG. 2, and many variations and modifications can be made toembodiments of the disclosure. In some other embodiments, only onediffractive grating structure is formed on one of the opposing surfacesof the first and second substrates 5A and 5B.

In some embodiments, the outer surfaces of the first and secondsubstrates 5A and 5B are respectively coated with an anti-reflectioncoating 7 to reduce the light reflection occurring thereon (i.e., toincrease the light transmittance of the diffractive optical element 4).Although not shown, at least one non-transparent coating is furtherprovided to cover a portion (e.g., the annular peripheral portion) of atleast one of the opposing surfaces of the first and second substrates 5Aand 5B to serve as an optical stop, thereby defining an optical aperture(i.e., a transparent window) of the diffractive optical element 4 thatallows light to pass through.

In some embodiments, the adhesive layer 8 (e.g., an adhesive gel) isdisposed between the annular peripheral portion of the opposing surfacesS1 and S2 of the first and second substrates 5A and 5B to bond the firstand second substrates 5A and 5B together. As such, a (vertical) gap G(or bonding gap G) is formed between the opposing surfaces S1 and S2 ofthe first and second substrates 5A and 5B. The size of the gap G mayvary with the thickness T of the adhesive layer 8 due to variousapplications and/or manufacturing tolerances.

Note that the size of the gap G may be accurately determined and thepower of the light source 2 may be adjusted accordingly before using theoptical pattern projection system 1 (FIG. 1), so that the generateddiffracted light beams L3 can be projected onto the object O with thedesired optical pattern of high quality (e.g., good resolution) andwell-controlled intensity (this is also about safety considerationsduring use, such as, in the facial recognition application). Forexample, usually, the power of the light source 2 may be lowered in thecase of a smaller size of the gap G in the diffractive optical element4, or the power of the light source 2 may be increased in the case of agreater size of the gap G in the diffractive optical element 4, in orderto maintain the intensity of the diffracted light beams L3 within anappropriate range. Consequently, it is needed to accurately obtain ordetermine the size of the gap G between the two substrates of thediffractive optical element 4 before use.

FIG. 3 is a schematic view of an optical test system 30 for determiningthe size of a gap between two substrates of an optical element, inaccordance with some embodiments. The optical test system 30 isconfigured to determine the size of a gap between two substrates of adiffractive optical element or other types of optical element (e.g., arefractive optical element) based on the operations of an optical testmethod 40 (FIG. 4) described below, in accordance with some embodiments.The following disclosure merely illustrates an example where the opticaltest system 30 (utilizing the optical test method 40) is used todetermine the size of a gap G between two substrates (such as the firstand second substrates 5A and 5B shown in FIG. 2) of a diffractiveoptical element (such as the diffractive optical element 4 shown in FIG.2), for the purpose of simplicity and clarity.

As shown in FIG. 3, the optical test system 30 includes a light source31, a holding stage 32, a sensor 33, a driving mechanism 34, and acontroller 35. Additional features can be added into the optical testsystem 30, and some of the features described below can be replaced oreliminated in other embodiments of the optical test system 30.

The light source 31 is configured to emit light that will pass throughan optical element during the test. In some embodiments, the lightsource 31 is a laser source configured to emit a laser beam L1 (e.g., avisible, IR, or other radiation). In some embodiments, the light source31 is used to emit the laser beam L1 in a wavelength or wavelength rangesimilar to (or the same as) that of the laser beam L1 (emitted by thelight source 2) being used in the optical pattern projection system 1for various applications. In some other embodiments, the light source 31may be another type of light source (e.g., a light-emitting diode(LED)).

The operations (e.g., activation, stop, and power control etc.) of thelight source 31 may be controlled by the controller 35 (e.g., acomputer). In some embodiments, the controller 35 can be a computerdevice including a processing unit and a memory device 351. Theprocessing unit can be implemented in numerous ways, such as withdedicated hardware, or with general-purpose hardware (e.g., a singleprocessor, multiple processors or graphics processing units capable ofparallel computations, etc.) that is programmed using microcode orsoftware instructions to perform the functions recited herein.

The laser beam L1 from the light source 31 is modulated by a collimatorlens 36 (FIG. 3) and a parallel collimated beam L2 is output from thecollimator lens 36 to pass through an optical element being tested. Insome other embodiments, the collimator lens 36 may also be omitted.

The holding stage 32 is disposed between the collimator lens 36 (or thelight source 31) and the sensor 33, configured to hold an opticalelement being tested. In some embodiments, the holding stage 32 isconfigured to hold a wafer W (FIG. 3) including a number of opticalelements (i.e., the optical elements are in wafer form prior to beingcut into several individual dies). In some embodiments, each opticalelement in the wafer W may be a diffractive optical element, such as thediffractive optical element 4 shown in FIG. 2. When the collimated beamL2 or the laser beam L1 passes through one of diffractive opticalelements 4 in a specified location of the wafer W mounted on the holdingstage 42, it is diffracted by the diffractive grating structures (suchas the first and second diffractive grating structures 6A and 6B shownin FIG. 2) in the diffractive optical element 4 into a number ofdiffracted light beams L3.

In some embodiments, the holding stage 32 includes a base 321 and aholder 322, as shown in FIG. 3. The base 321 is stationary part in theoptical test system 30. The holder 322 is a movable part movablydisposed on the base 321 and configured to hold, for example, the waferW (or the diffractive optical element 4) in some embodiments. During thetest, the driving mechanism 34 is configured to drive the holder 322with the wafer W (or the diffractive optical elements 4) to moverelative to the base 321, for example, in a first direction D1 (FIG. 3)substantially parallel to the direction of light propagation (i.e.,perpendicular to the light receiving surface (such as the lower surfaceshown in FIG. 3) of the sensor 33) in the optical test system 30.

In some embodiments, the driving mechanism 34 includes a number ofscrews 341, a number of nuts 342, and a number of (rotary) drivingmotors 343, as shown in FIG. 3. One nut 342 and one screw 341 may becorrespondingly coupled together and form a lead screw that can converta rotational movement of the screw 341 into a linear movement (along thefirst direction D1) of the nut 342. The driving motor 343 is configuredto drive the screw 341 to rotate. In some embodiments, the (four)corners of the (square) base 321 of the holding stage 32 arerespectively provided with a screw 341, a nut 342, and a driving motor343. With these configurations, the driving mechanism 34 drives theholder 322 with the wafer W (or the diffractive optical element 4) tomove relative to the base 321 in the first direction D1 during the test.In some other embodiments, the driving mechanism 34 may include singledriving motor 343 to drive one screw 341 to rotate, so that thecorresponding nut 342 and the holder 322 move along the screw 341.

The sensor 33 is configured to receive the light passing through theoptical element during the test and generate an intensity (electric)signal in response to the intensity of the received light. In someembodiments, the sensor 33 is configured to selectively receive one ofthe diffracted light beams L3 (which will be further illustrated later)from the diffractive optical element 4, and capable of generating anintensity signal in response to the intensity of the received diffractedlight beam L3. In some embodiments, the sensor 33 is a charge coupleddevice (CCD), a complementary metal-oxide semiconductor (CMOS) sensor orthe like. The intensity signal generated by the sensor 33 is sent to thecontroller 35 for further processing (which will be illustrated later).

In some embodiments, the optical test system 30 further includes adistance meter 37 (e.g., a laser interferometer) configured to measureor determine the displacement or position of the holder 322 in theoptical test system 30, as shown in FIG. 3. The distance meter 37 isalso capable of generating a position signal in response to the positionof the holder 322 and sending the position signal to the controller 35.The controller 35 controls the aforementioned operations of the drivingmechanism 34 according to the position signal from the distance meter37, and a computer program, related to the control of the displacement(which will be further illustrated later) of the holder 322 (i.e., thecontrol of the operation of the driving mechanism 34) during the test,stored in a memory device 351 (e.g., a random access memory (RAM), aread-only memory (ROM) or the like) of the controller 35.

Next, referring to FIG. 4, which is a simplified flowchart of an opticaltest method 40 (implemented by the optical test system 30 shown in FIG.3) for determining the size of a gap between two substrates of anoptical element (for example and without limitation, a diffractiveoptical element 4 as shown in FIG. 2) before use, in accordance withsome embodiments. For illustration, the flow chart will be describedalong with the drawings shown in FIGS. 2-3 and 5-8. Some of thedescribed operations can be replaced or eliminated in differentembodiments. Alternatively, some operations may be added in differentembodiments. The optical test method 40 includes a number of operations,such as operations 41, 42, 43, 44, and 45.

In operation 41, before determining the size of the gap G (FIG. 2) in adiffractive optical element 4 (for illustration, hereinafter alsoreferred to as a “tested optical element 4”), a number of opticalelement models M are placed on the movable holder 322 (FIG. 5) in theoptical test system 30 to be tested as described below.

In some embodiments, each of the optical element models M is also adiffractive optical element (the same as the tested optical element 4)having a gap between two substrates thereof, and the sizes of the gapsof the optical element models M are different (i.e., the optical elementmodels M have different bonding gap sizes). In some embodiments, theoptical element models M have the same optical structure configurationas the tested optical element 4. For example, the optical element modelsM and the tested optical element 4 have the same configuration andarrangement of the first and second diffractive grating structures, aswell as the same thickness of the first and second substrates and othermaterial layers (except the adhesive layer) in a diffractive opticalelement as described above (FIG. 2). In some embodiments, the opticalelement models M are formed by the same fabrication processes as thetested optical element 4.

Next, in operation 42, the optical test system 30 is operated to collectmultiple sets of intensity signals (for illustration, hereinafter alsoreferred to as multiple sets of “sample intensity signals”) that arecorresponding to the optical element models M (with the differentbonding gap sizes) as reference data for the tested optical element 4.

In some embodiments, the multiple sets of sample intensity signalscorresponding to the optical element models M are measured and collected(which will be illustrated later) by the optical test system 30 whilethe optical element models M are in wafer form during the test. In someembodiments, the optical element models M (with the different bondinggap sizes) are in the same wafer W′ or different wafers W′ and beingdisposed on the holder 322 during the test (FIG. 5).

In some embodiments, in order to generate one set of sample intensitysignals that is corresponding to one of the optical element models M(with a specified bonding gap size), the driving mechanism 34 (FIG. 3)drives the holder 322 with the wafer W′ (or the optical element model M)to move (relative to the base 321 shown in FIG. 3) in the firstdirection D1 to multiple positions during the test. At the same time,the light source 31 emits light to pass though the optical element modelM to generate a number of diffracted light beams L3, and the sensor 33selectively receives one of the diffracted light beams L3 (e.g., thecircled diffracted light beam L3 shown in FIG. 5)). The operation of thedriving mechanism 34 may be controlled by the controller 35 (FIG. 3) insome embodiments.

FIG. 5 is a schematic view illustrating that the holder 322 (with theoptical element model M) is moved to multiple positions (e.g., fourpositions P1, P2, P3, and P4; where the distances of the holder 322 (orthe optical element model M) to the sensor 33 are indicated by X1, X2,X3, and X4, respectively) during the test, in accordance with someembodiments. In some embodiments, the positions P1, P2, P3, and P4 ofthe holder 322 are apart from the original position P0 of the holder 322(e.g., where the holder 322 is closet to the base 321) by 50 μm, 100 μm,150 μm, and 200 μm, respectively. That is, the distances between twoadjacent positions in the four positions P1, P2, P3, and P4 may be thesame. In some alternative embodiments, the holder 322 with the wafer W′(or the optical element model M) may also be driven by the drivingmechanism 34 to move in the first direction D1 to N positions during thetest, wherein N is a natural number greater than or equal to 2 and thedistances between two adjacent positions in the N positions are the sameor different, in some other embodiments. It should be realized that theminimum distance between two adjacent positions of the holder 322depends on the movement limit of the holder 322, and the maximumdistance between two adjacent positions of the holder 322 is chosen sothat the sensor 33 can successfully receive intensity signals from theoptical element model M at all moving positions (i.e. the movement ofthe optical element model M does not exceed the focus range of thesensor 33).

Therefore, the distance between the optical element model M and thesensor 33 may vary along with the different positions of the holder 322,and also the intensity of the diffracted light beam L3 (received by thesensor 33) from the optical element model M may vary accordingly.

It should also be understood that the intensity of the diffracted lightbeam from a diffractive optical element (e.g., the optical element modelM) is given by the sinc function of the distance of the positive ornegative diffracted light beam to the 0-order diffracted light beam onthe object (or the sensor 33) and the distance of the diffractiveoptical element to the object/the sensor 33 (i.e., the Huygens-Fresnelprinciple). As such, the intensity of the positive or negativediffracted light beam (e.g., +1-order, −1-order, or a higher orderpositive or negative diffracted light beam) from the optical elementmodel M received by the sensor 33 may vary along with the differentpositions of the optical element model M (i.e., different distances ofthe optical element model M to the sensor 33). Conversely, the intensityof the 0-order diffracted light beam from the optical element model Mreceived by the sensor 33 does not vary along with different positionsof the optical element model M.

Accordingly, by receiving one of the positive and negative diffractedlight beams, such as +1-order or −1-order diffracted light beam (e.g.,the circled diffracted light beam L3 shown in FIG. 5) from the opticalelement model M in N positions, the sensor 433 generates N sample(second) intensity signals (i.e., one set of sample intensity signals)that are corresponding to the optical element model M in the Npositions. In this case, N is greater than or equal to 3 to better andaccurately express the optical element model M with a specified bondinggap size between two substrates thereof by the N sample (second)intensity signals.

In operation 42, multiple sets of sample (second) intensity signals(i.e., the reference data) corresponding to a number of optical elementmodels M (with different bonding gap sizes) may also be generated by thesensor 33 and then sent to the controller 35, and the memory device 351of the controller 35 stores the reference data, in some embodiments.Additionally, after collecting the multiple sets of sample intensitysignals corresponding to the optical element models M, the wafer(s) W′(or the optical element models M) may be removed from the holder 322 toa microscope to measure the size of a gap between two substrates of eachof the optical element models M. In some embodiments, the sizes of thegaps of the optical element models M may be determined by observing thecross-sections of the sliced optical element models M.

FIG. 6 is a diagram plotting multiple sets (e.g., four sets) of sampleintensity signals (respectively including four sample (second) intensitysignals) that are corresponding to a number of optical element models M(with the different bonding gap sizes) in multiple positions (e.g., fourpositions corresponding to the positions P1, P2, P3, and P4 asillustrated in the embodiments of FIG. 5), in accordance with someembodiments. As shown in FIG. 6, the intensity of multiple sets ofsample (second) intensity signals corresponding to the optical elementmodels M with different bonding gap sizes is different.

In some embodiments, the sample (second) intensity signals correspondingto one of the optical element models M in N different positions mayfurther form a characteristic curve corresponding to the optical elementmodel M if the number (N) of sample intensity signals is large enough,as shown in FIG. 6. In some embodiments, the sample (second) intensitysignals corresponding to the optical element models M in the originalposition P0 (FIG. 5) may also be used as the reference data. In someembodiments, the total displacement amount of the holder 322 (with theoptical element models M) relative to the base 321 may also be adjustedor changed.

In operation 43, after the reference data (such as the diagram shown inFIG. 6) for the tested optical element 4 is collected and the opticalelement models M are removed from the holder 322, the tested opticalelement 4 is placed on the holder 322 to be tested as described below.

Next, in operation 44, the optical test system 30 is operated to measureN (first) intensity signals corresponding to the tested optical element4 in the N positions that are the same as the N positions where thesample (second) intensity signals corresponding to the optical elementmodels M are measured and collected. In some alternative embodiments,the optical test system 30 is operated to measure N′ (first) intensitysignals corresponding to the tested optical element 4 in N′ positions,while N′ is a natural number greater than 2 and less than N, and the N′positions correspond to some of the N positions.

In some embodiments, similar to the operation 42 for measuring themultiple sets of sample intensity signals corresponding to the opticalelement models M, the driving mechanism 34 (FIG. 3) drives the holder422 with the tested optical element 4 to move (relative to the base 321shown in FIG. 3) in the first direction D1 to N positions (e.g., fourpositions P1, P2, P3, and P4) during the test of the tested opticalelement 4, as shown in FIG. 7. At the same time, the light source 31emits light to pass though the tested optical element 4 to generate anumber of diffracted light beams L3, and the sensor 33 selectivelyreceives one of the diffracted light beams L3 (e.g., the circleddiffracted light beam L3 shown in FIG. 7)) from the tested opticalelement 4. In some embodiments, the received diffracted light beam L3 isa positive or negative diffracted light beam (e.g., +1-order or −1-orderdiffracted light beam), the same as the received diffracted light beamL3 for generating the sample (second) intensity signals in the operation42. In addition, the four positions P1, P2, P3, and P4 may be the sameas those for measuring and collecting the sample (second) intensitysignals in the operation 42.

Accordingly, the sensor 33 generates N (first) intensity signals (e.g.,four first intensity signals as the cross marks indicate in FIG. 8)corresponding to the tested optical element 4 in the N positions.

In operation 45, the size of a gap between two substrates of the testedoptical element 4 is determined according to the measured N (first)intensity signals and the previously collected reference data (includingmultiple sets of sample (second) intensity signals corresponding to anumber of optical element models M) stored in the memory device 351 ofthe controller 35.

In some embodiments, in order to determine the size of the gap betweentwo substrates of the tested optical element 4, the controller 35 isused to calculate the intensity difference (absolute value) between themeasured N (e.g., four) first intensity signals and the N (e.g., four)second intensity signals of each set of sample intensity signalscorresponding to a number of sample optical elements M. Based on thecalculation or comparison result, the controller 35 determines that thesize of the gap between two substrates of the tested optical element 4is the same as that of one of the optical element models M (such as theoptical element model M with the gap size of 3^(rd) value μm as shown inFIG. 8) when the sum of the intensity difference between the N firstintensity signals and the N second intensity signals corresponding tothe optical element model M is the minimum (with respect to the otheroptical element models M). Consequently, the size of the gap between twosubstrates of the tested optical element 4 is determined.

In some embodiments, after the gap size of one of the tested opticalelement 4 in the wafer W is determined, the holder 322 may also be movedby a driving mechanism (not shown) in a second direction D2 (FIG. 3)perpendicular to the first direction D1 so that another tested opticalelement 4 in the wafer W is moved to the tested position where the lightbeam from the light source 41 will pass through. Afterwards, the aboveoperations of the optical test method 40 are repeated for determiningits gap size. In some embodiments, the sensor 33 of the optical testsystem 30 is also configured to measure the intensity of the diffractedlight beams from a number of or all the tested optical elements 4 in thewafer W at one time.

The embodiments of the present disclosure have some advantageousfeatures: The size of the gap between two substrates of an opticalelement (e.g., a diffractive optical element or another type of opticalelement) can be accurately determined by the optical test system usingthe optical test method as described above, without using an electricaltest method which needs to provide electrical components (e.g.,conductive layers and pads) in the optical element for the electricaltest implemented by an electrical testing apparatus. Accordingly, thenumber of operations in the process of forming the optical element, aswell as the cost of the electrical testing apparatus, may be reduced.

In some embodiments, an optical test method is provided. The opticaltest method includes emitting light through a gap between two substratesof a tested optical element disposed on the holder to generate aplurality of light beams. The optical test method further includesdriving the holder with the tested optical element to move to Npositions. The optical test method also includes receiving one of thelight beams from the tested optical element in the N positions togenerate N first intensity signals. In addition, the optical test methodincludes determining the size of the gap of the tested optical elementaccording to the N first intensity signals and reference data.

In some embodiments, an optical test method is provided. The opticaltest method includes collecting multiple sets of sample intensitysignals corresponding to a plurality of optical element models asreference data. The optical element models respectively have a gapbetween two substrates of each of the optical element models, and thesizes of the gaps of the optical element models are different. Each setof sample intensity signals comprises N second intensity signalscorresponding to one of the optical element models in N positions. Theoptical test method further includes measuring N first intensity signalscorresponding to a tested optical element in the N positions. Inaddition, the optical test method includes determining the size of a gapbetween two substrates of the tested optical element according to the Nfirst intensity signals and the reference data.

In some embodiments, an optical test system is provided. The opticaltest system includes a holder, a light source, a driving mechanism, asensor, and a controller. The holder is configured to hold an opticalelement. The light source is configured to emit light through theoptical element to generate a plurality of light beams. The drivingmechanism is configured to drive the holder with the optical element tomove N positions. The sensor is configured to receive one of the lightbeams from the optical element in the N positions to generate N firstintensity signals. The controller is configured to determine the size ofa gap between two substrates of the optical element according to the Nfirst intensity signals and reference data.

Although embodiments of the present disclosure and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims. For example, it will be readily understood by those skilled inthe art that many of the features, functions, processes, and materialsdescribed herein may be varied while remaining within the scope of thepresent disclosure. Moreover, the scope of the present application isnot intended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present disclosure,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present disclosure. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.In addition, each claim constitutes a separate embodiment, and thecombination of various claims and embodiments are within the scope ofthe disclosure.

What is claimed is:
 1. An optical test method, comprising: emittinglight through a gap between two substrates of a tested optical elementdisposed on a holder to generate a plurality of light beams; driving theholder with the tested optical element to move to N positions, wherein Nis a natural number greater than 2; receiving one of the plurality oflight beams from the tested optical element in the N positions togenerate N first intensity signals; and determining the size of the gapof the tested optical element according to the N first intensity signalsand reference data.
 2. The optical test method as claimed in claim 1,wherein the reference data comprises multiple sets of sample intensitysignals corresponding to a plurality of optical element models whichrespectively have a gap between two substrates of each of the opticalelement models, the sizes of the gaps of the optical element models aredifferent, and each set of sample intensity signals comprises N secondintensity signals corresponding to one of the optical element models inthe N positions.
 3. The optical test method as claimed in claim 2,wherein the operation of determining the size of the gap of the testedoptical element further comprises: calculating intensity differencesbetween the N first intensity signals and the N second intensity signalsof each set of sample intensity signals; and determining that the sizeof the gap the tested optical element is the same as that of one of theoptical element models when a sum of the intensity differences betweenthe N first intensity signals and the N second intensity signalscorresponding to the one of the optical element models is the minimum.4. The optical test method as claimed in claim 1, wherein N is greaterthan or equal to
 3. 5. The optical test method as claimed in claim 1,wherein the holder with the tested optical element is driven to move bya plurality of driving motors.
 6. The optical test method as claimed inclaim 1, wherein the tested optical element is a diffractive opticalelement.
 7. The optical test method as claimed in claim 6, wherein thereceived light beam for generating the first intensity signal is apositive or negative diffracted light beam from the diffractive opticalelement.
 8. The optical test method as claimed in claim 6, wherein thediffractive optical element comprises a first substrate and a secondsubstrate stacked on each other, at least one diffractive gratingstructure is formed on at least one of opposing surfaces of the firstand second substrates, and the gap is formed between the opposingsurfaces of the first and second substrates.
 9. An optical test method,comprising: collecting multiple sets of sample intensity signalscorresponding to a plurality of optical element models as referencedata, wherein the optical element models respectively have a gap betweentwo substrates of each of the optical element models, the sizes of thegaps of the optical element models are different, and each set of sampleintensity signals comprises N second intensity signals corresponding toone of the optical element models in N positions, wherein N is a naturalnumber greater than 2; measuring N first intensity signals correspondingto a tested optical element in the N positions; and determining the sizeof a gap between two substrates of the tested optical element accordingto the N first intensity signals and the reference data.
 10. The opticaltest method as claimed in claim 9, wherein the step of determining thesize of a gap between two substrates of the tested optical elementfurther comprises: calculating intensity differences between the N firstintensity signals and the N second intensity signals of each set ofsample intensity signals; and determining that the size of the gapbetween two substrates of the optical element is the same as that of oneof the optical element models when a sum of the intensity differencesbetween the N first intensity signals and the N second intensity signalscorresponding to the one of the optical element models is the minimum.11. The optical test method as claimed in claim 9, further comprisingmoving a holder holding the tested optical element in a first directionto the N positions during the measuring, wherein the first direction isperpendicular to a light receiving surface of a sensor.
 12. The opticaltest method as claimed in claim 9, wherein the distances between twoadjacent positions in the N positions are the same.
 13. An optical testsystem, comprising: a holder configured to hold an optical element; alight source configured to emit light through the optical element togenerate a plurality of light beams; a driving mechanism configured todrive the holder with the optical element to N positions; a sensorconfigured to receive one of the light beams from the optical element inthe N positions to generate N first intensity signals; and a controllerconfigured to determine the size of a gap between two substrates of theoptical element according to the N first intensity signals and referencedata.
 14. The optical test system as claimed in claim 13, wherein thereference data comprises multiple sets of sample intensity signalscorresponding to a plurality of optical element models whichrespectively have a gap between two substrates of each of the opticalelement models, the sizes of the gaps of the optical element models aredifferent, and each set of sample intensity signals comprises N secondintensity signals corresponding to one of the optical element models inthe N positions
 15. The optical test system as claimed in claim 13,wherein the controller further comprises a memory device configured tostore the reference data.
 16. The optical test system as claimed inclaim 13, wherein the driving mechanism comprises at least one drivingmotor.
 17. The optical test system as claimed in claim 13, furthercomprising a holding stage comprising the holder and a base, wherein theholder is movably disposed on the base, and the driving mechanism isconfigured to drive the holder to move relative to the base in a firstdirection perpendicular to a light receiving surface of the sensor 18.The optical test system as claimed in claim 13, wherein the lightemitted from the light source is infrared light.
 19. The optical testsystem as claimed in claim 13, wherein the optical element is adiffractive optical element.
 20. The optical test system as claimed inclaim 19, wherein the light beam received by the sensor for generatingthe first intensity signal is a positive or negative diffracted lightbeam from the diffractive optical element.